A revolution in gameplay? Hardly. Rather, it is a demonstration that existing and well-established genres can be adapted to a new business environment.
If the casual downloadable market is the measure of what constitutes a "casual game," then social network games are quite different. The genres that are successful in the two markets are different, the 60 minute demo is out the window, and while demographics are similar, they're not identical - social games are younger and more male than the downloadable market, though still heavily female.
Basically, social network games are being called "casual" only because many developers and businesses have a direct stake in promoting the meme that casual is different and new and exciting and venture capitalists should give them money now.
Let's take a step back.
Every kind of game is shaped by the aesthetic of its audience. To talk of "gamers" is almost a misnomer, because gamers come in many stripes. What an FPS player wants from a game is very different from what a tabletop RPG player wants, and that's very different from what an adventure gamer wants. To put it another way: to talk of "casual" and "hardcore" games is to fail to understand how and why people like games. There are not two kinds of games, or two kinds of gamers; there are dozens of kinds of games, and hundreds of kinds of gamers. And every kind of game can support both "casual" and "hardcore" play; some will fire up an FPS and have a deathmatch or two to relax, and I have no doubt there are people who play match-three games for dozens of hours weekly and work hard to improve their high score.
Just as audience aesthetic shapes a genre, so do its business constraints. Conventional videogames cost $60 or so at retail, at least when new, and to support that price point, necessarily have to offer dozens of hours of gameplay, and a deep, rich experience. This crowds out puzzle games, adventure games, and other genres that have many fans - anything "light" in fact. The great virtue of the growth of alternative distribution channels is not that "now we can have casual games," but rather that "now there are viable ways to distribute puzzle games, light RPGs, and other neglected genres."
The whole idea of the "casual" game is not only meaningless; it's also counterproductive. It's useless to think thoughts like "How do I appeal to casual gamers?" What's useful is to think thoughts like, "What is the nature of this distribution channel, and what kinds of games can succeed here that don't succeed at conventional retail?" and "What is the aesthetic of this audience, and how do I appeal to it?" and "What can be borrowed from other game styles and adapted to this new environment?"
The idea that we now have "casual" games that are "games for the rest of us" is bogus; instead, we are now all gamers. Everyone under 50 has been exposed to games of one sort or another, and everyone except for the very oldest portion of the population will play games, if games that appeal to their interests are made accessible to them. And "we" are diverse, ornery, and individually weird. There are huge opportunities, now, because new distribution channels and new ways of reaching an audience are opening up; but the answer is not a one-size-fits-all, mass audience, "casual" solution; the answer is a let-a-thousand-flowers-blossom, exploit-every-niche, be-creative, hardcore solution.